[GUI/Post-vis] NEW FEATURE: Enable image overlay | BUG fix: set detault settings in health status to prevent execption

New feature:

One can now add images in the post-vis:

Several images are provided:

The user can choose any combination of images (or simply one image):

screencast

Bug fix:

Class: SimulationRenderer -> case HEALTH_STATUS (Problem: null pointer because no defaults were set)

case HEALTH_STATUS: {
                if (agent instanceof Pedestrian) {
                    Pedestrian pedestrian = (Pedestrian) agent;

                    boolean isInfectious = false; // NEW set defaults here
                    double degreeOfExposure = 0.0;  // NEW set defaults here
                    if (pedestrian.getHealthStatus() != null) {
                        isInfectious = pedestrian.isInfectious();
                        degreeOfExposure = pedestrian.getDegreeOfExposure();
                    }
                    return model.config.getHealthStatusColor(isInfectious, degreeOfExposure);
           }